This invention relates to hydraulic circuit apparatus for construction machines, such as hydraulic excavators, hydraulic cranes, etc., and more particularly to a control system for a hydraulic circuit apparatus for controlling the speeds of actuators by the displacement volumes of hydraulic pumps.
Nowadays in hydraulic circuit apparatus for civil engineering and construction machines, such as hydraulic excavators, hydraulic cranes, etc., speeds of the actuators are controlled by the displacement volumes of variable displacement hydraulic pumps. For example, in a hydraulic excavator, a plurality of variable displacement type hydraulic pumps are connected in closed or semi-closed circuit with actuators for driving working elements, such as a boom, an arm, a bucket, a pair of tracks and a swing, so as to control the speeds and directions of movements of the actuators by the displacement volumes and directions of the hydraulic pumps. Even when the hydraulic pumps are connected with the actuators in open circuit, the speeds of the actuators are controlled by the displacement volumes of the hydraulic pumps to conserve energy.
In this type of hydraulic circuit apparatus, proposals have been made to use a circuit apparatus including at least first and second hydraulic pumps of the variable displacement type, a first hydraulic actuator arranged for hydraulic connection with the first pump through first valve means to be driven thereby, and a second hydraulic actuator arranged for selective hydraulic connection with the first and second pumps through second and third valve means respectively to be driven thereby. In a control system for this hydraulic circuit apparatus, the order of priority for hydraulic connection is set beforehand in such a manner that when an operation signal for the second actuator is received while the first pump is inoperative, the first pump takes priority over the second pump for hydraulic connection with the second actuator, and when an operation signal for the first actuator is received while the first pump is in hydraulic connection with the second actuator, the first actuator takes priority over the second actuator for hydraulic connection with the first pump and the second actuator is brought into hydraulic connection with the second pump. The displacement volume of the first pump and switching of the second valve means are controlled in such a manner that when the first pump which is in hydraulic connection with the second actuator is to be brought into hydraulic connection with the first actuator, the displacement volume of the first pump is once returned to zero before changing of the hydraulic connection. Also, the displacement volume of the second pump and switching of the third valve means are controlled in such a manner that hydraulic connection between the second actuator and the second pump takes place when the first pump is switched from the second actuator to the first actuator for hydraulic connection.
Thus, if an operation signal for the first actuator is supplied when the first pump is in hyraulic connection with the second actuator, then the displacement volume of the first pump is first returned to zero, and when the volume has become zero, the second actuator is switched from the first pump to the second pump for hydraulic connection while the second pump starts its displacement, so that the inflow of the hydraulic fluid into the second actuator shows a change. This causes a change in the speed of the second actuator to occur, thereby influencing operability. Particularly when the second actuator is a swing motor or track motors, the brake is temporarily applied thereto and trouble may occur.
Furthermore, when the displacement volume of the first pump is first returned to zero, it is necessary that the displacement volume have a rate of change such that the change takes place gradually so as not to give a shock to the working elements or machines driven by the second actuator. Thus, the time elapsing after a decrease in the displacement volume of the first pump is initiated until it reaches zero is relatively long, so that it takes a considerably long period of time for the first actuator to be brought into hydraulic connection with the first pump and driven thereby after an operation signal for the first actuator is supplied.